drive D is more likely to be the CD/dvd drive bryan
whats old HD size? and whats size of new one?

you need to get the case open to check you have a slot free for the extra hard drive if there is the space then change the pin settings so the new drive is a slave and then connect the power plug (prob tied up somewhere) and ide ribbon cable that will be connected to your existing drive may have a extra connector in series that you can then use to connect to the slave drive.

if its just a case of running out of hard drive space then back up large files pictures ect onto the external hard drive as a habit.
what i do is regularly copy/paste my whole picture,movies/music folders onto the external drive and it just updates with any new files.
then worst case scenario you need to wipe computer you can get all your stuff back on easily.

can pm u my home number if you want talking through it mate